uboot: unbreak legal info for legacy versions

Fixes #6986

Prior to u-boot 2013.10, the GPL license text was stored in COPYING and not
Licenses/gpl-2.0.txt, breaking legal-info.

Work around it by simply copying the file from the old location to the new
if present.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>